Respiratory Monitoring: Physiological and Technical Considerations

Nevner mye viktig om oksygen metning og oximetri. Sier at den normal intracellulære oxygenmetninger er 40 mmHG, som er en SpO2 på 80%.

A useful caveat is to memorize the significance of 2 saturation readings; 95% and 90%. An SpO2 of 95 reflects a PaO2 of ~80 mm Hg. This is by definition the lower limit of normal oxygenation. The precise oxygen tensions for saturations above 95 are irrelevant; they reflect normal PaO2. An SpO2 of 90 reflects a PaO2 of ~60 mm Hg. By definition, this is hypoxemia, but it is well above the normal intracellular oxygen tension of 40 mm Hg.

More importantly, 90% saturation represents a critical point on the oxygen-hemoglobin dissociation curve (See Figure 1). Further decline leads to a precipitous drop in saturation, thus oxygenation, as hemoglobin loses oxygen rapidly. Saturation that declines to 90% is at the “edge of the cliff” and is a warning to aggressively reestablish adequate ventilation. PaO2 extrapolations no longer require memorization because they are ~30 less than the SpO2 reading. For example, if hemoglobin saturation is 83%, the PaO2 is ~53 mm Hg.

While PaO2 is used to assess oxygenation, PaCO2 is the true measure of ventilation.

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